This is not Hostinger support, just the forum for AMP. We can only help you with panel related issues, and cannot install a modpack for you.
There is a FAQ here:
https://discourse.cubecoders.com/t/forge-frequently-asked-questions/2982
You need to make sure that you are uploading the server pack zip, and not the mod manifest from the client. You can then unzip the server pack in the web file manager.